Digital transmission systems have become familiar in daily life with progress in fiber optic cable or LSI manufacturing techniques and digital signal processing techniques. Optical communication methods permit large scale, long distance relays with little communication loss, and far less long distance communication cost. Moreover, the bandwidth required for digital data transmission has been suppressed due to progress of coding techniques. Hence, ISDN (Integrated Services Digital Network) services which can handle multimedia, such as telephone, data and images in a unified way, were started, and SDH (Synchronous Digital Hierarchy) which is a mass communication-oriented multiplexing method, was standardized in a form which supports generalization of ISDN. SDH is a globally universal, fully synchronous interface which can be expanded from high capacity communications to the telephone network, which permits flexible multiplexing of various communications service data which has ample operating and maintenance features, and is highly reliable. SDH with these characteristics are now being constructed all over the world, and are employed as the main technique in physical layers in many networks even today with the spread of the Internet.
The Internet, which has become a worldwide phenomenon in recent years, is an information network which is based on IP (Internet Protocol). IP can use address dynamic setting functions, such as routing protocols, e.g., OSPF and DHCP, and requires little effort for network construction. Therefore, IP is rapidly becoming universal and is already used as a global information network. IP networks provided by carriers and ISP are indispensable to life today. In recent years, as regards data link layers, the use of LAN (Local Area Networks) in companies or households has also been progressing, and information and telecommunications networks which use Ethernet (registered trademark) are being constructed due to their ease of handling. Examples are wide area Ethernet services and metro Ethernet services. As can be seen from this situation, if everyone can use and build a network, it will require less time to make network connections, and there will be an increasing shift to a connection-less communication method which permits dynamic operation.
Reflecting such technological trends, the interfaces in routers or switches also generally use Ethernet. Ethernet is not strictly equipped with employment maintenance functions like SDH, and it is difficult to guarantee quality because of connection-less communication. However, since it has becoming more popular, standardization bodies are considering the implementation of data utility architecture by Ethernet and implementation of OAM in Ethernet. To deal with increasing amounts of information, studies are now in progress on Ethernet with throughputs of 10 Gbps and 40 Gbps, and a gradual shift to asynchronous networks is taking place.
However, since SDH has been widely used in carrier networks or company networks, under the present circumstances, since network shifts take time, and since it is still not as reliable as fully synchronous networks in Ethernet or IP base communication technology, it is expected that synchronous networks and asynchronous networks will continue to co-exist for some time. Although Ethernet frames are being forwarded on SDH frames, mutual communication functions with an Ethernet physical layer and a synchronous network including SDH, have hardly been provided until now. Hence, interconnection techniques which make use of conventional resources will be indispensable to the transition period to asynchronous networks.
Due to the expansion of connection-less communication, the service types which can be used on a network are increasing, and more and more services are demanding network quality, not only for perusing Web data but also for voice telephone, video-on-demand and access to confidential data, etc. Although they are conflicting requirements, user convenience, and the maintenance of communication quality and security, will be important matters to consider in future network construction.